either unset RUST_BACKTRACE or set it to: "0"|"n"|"no"|"off" (case-sensitive though)
to get the same effect!
Some programs(vim's syntactic used by rust.vim) cannot unset this! but can set it to some value, and I needed this to avoid issue: https://github.com/rust-lang/rust/issues/29293

see PR: https://github.com/rust-lang/rust/pull/32549

diff --git a/src/libstd/sys/common/backtrace.rs b/src/libstd/sys/common/backtrace.rs
index 3c84783..2775016 100644
--- a/src/libstd/sys/common/backtrace.rs
+++ b/src/libstd/sys/common/backtrace.rs
@@ -36,7 +36,8 @@ pub fn log_enabled() -> bool {
     }
 
     let val = match env::var_os("RUST_BACKTRACE") {
-        Some(..) => 2,
+        Some(x) => if &x == "0" || &x == "n"
+            || &x == "no" || &x == "off" { 1 } else { 2 },
         None => 1,
     };
     ENABLED.store(val, Ordering::SeqCst);
